Electrical submersible pumps are often used for pumping a mixture of oil and water from a well. Normally the pump assembly has an electrical motor and a rotary pump, which may be centrifugal or other types. The motor is filled with a dielectric lubricant, and a seal section between the motor and the pump serves to equalize the internal pressure of the lubricant with the hydrostatic pressure on the exterior of the pump assembly.
A typical seal section, also called a pressure equalizer, has a tubular housing through which a drive shaft extends for transmitting rotation of the motor to the pump. A thrust bearing assembly is often located in the seal section for absorbing downthrust created by the pump. The lubricant in the pump also lubricates the thrust bearing.
Various means are employed to equalize lubricant pressure with the well fluid. A tubular elastomeric bladder may be mounted in the seal section, the bladder having an interior in fluid communication with the lubricant in the motor. A well fluid passageway allows well fluid to enter the seal section on the exterior of the bladder. Labyrinth tubes are also employed, either alone or in a separate chamber from the bladder. The water of the well fluid is normally denser than the oil. Generally, the labyrinth tubes are mounted with an upper inlet and a lower outlet, so that water flowing downward through the tube cannot flow back upward through the outlet in a manner so as to migrate into the motor.
The seal section also has features to accommodate expansion of the lubricant in the motor, which occurs as the motor gets hotter. A check valve may be employed to expel excess lubricant without allowing the entry of well fluid.
Most seal sections have multiple chambers, usually two to four, for housing the bladder and labyrinth tubes. Normally, each chamber is a cylindrical sleeve secured at its upper and lower ends by threads to adapters and shaft support members. The additional threaded sleeves add to the cost of a seal section.